Browsing by Author "Sethi, Shiv K."

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Constraints on omegaB, omega M, and h from MAXIMA and BOOMERANG
(2000-10-24) Padmanabhan, T.; Sethi, Shiv K.
We analyse the BOOMERANG and MAXIMA results in the context of models with ΩTotal = 1 and ns = 1. We attempt to constrain three other parameters—h, ΩB, and Ωm—from these observations. We show that: (a) the value of ΩBh2 is too high to be compatible with primordial nucleosynthesis observations at 95% conﬁdence level (b) universe with age greater than 12Gyr is ruled out at 95% conﬁdence level and (c) the value of Ωmh is too high to be compatible with the shape of the power spectrum of gravitational clustering. In eﬀect, our analysis shows that models with ΩTotal = 1 and n = 1 are ruled out by BOOMERANG and MAXIMA observations.
Constraints on ΩB, Ωm, and h from MAXIMA and BOOMERANG
(American Astronomical Society, 2001-07-01) Padmanabhan, T.; Sethi, Shiv K.
We analyze the BOOMERANG and MAXIMA results in the context of simplest inflationary universes: ΩTotal = 1, ns~ 1. We attempt to constrain three other parameters-h, ΩB, and Ωm from these observations. We show that (1) the data are consistent with the values of Ωm and h inferred from other observations and (2) the value of ΩBh² is too high to be compatible with big bang nucleosynthesis observations at the 2 σ level for ns = 1. We also include two cosmic background imager(CBI) band powers in our analysis. However, the inclusion of CBI band powers doesn't affect our conclusions.
On the interpretation of the He II absorption in the line of sight of Q0302-003
(2015-01-27) Nath, B. B.; Sethi, Shiv K.
We point out the peculiarities of the line of sight of Q0302-003 which was recently used to estimate the He II opacity at z∼3.285. This line of sight contains a large void in Lyman α clouds, in the region used for the He II opacity observation, and the void is thought to be caused by the ionizing radiation from a foreground quasar (Dobrzycki and Bechtold 1991). If this is true, then the radiation from the foreground quasar is more relevant in the region used for the estimation of the He II opacity than the diffuse UV background radiation. We argue that in that case the observed opacity should not be used to put constraint on the diffuse UV background radiation. Using a typical spectral index of quasars, we estimate that most of the observed He II opacity must be due to Gunn-Peterson effect and suggest a H I Gunn-Peterson test along this line of sight. We further discuss the clumpiness of the intergalactic medium in the vicinity of Q0302-003 as suggested by observations
Primordial magnetic fields and formation of molecular hydrogen
(2009-04-01) Sethi, Shiv K.; Nath, B. B.; Subramanian, Kandaswamy; et al.
We study the implications of primordial magnetic ﬁelds for the thermal and ionization history of the post-recombination era. In particular we compute the eﬀects of dissi- pation of primordial magnetic ﬁelds owing to ambipolar diﬀusion and decaying tur- bulence in the intergalactic medium (IGM) and the collapsing halos and compute the eﬀects of the altered thermal and ionization history on the formation of molecular hy- drogen.We show that, for magnetic ﬁeld strengths in the range 2×10−10 G < ∼ B0 < ∼ 2× 10−9 G, the molecular hydrogen fraction in IGM and collapsing halo can increase by a factor 5 to 1000 over the case with no magnetic ﬁelds. We discuss the implication of the increased molecular hydrogen fraction on the radiative transfer of UV photons and the formation of ﬁrst structures in the universe
Primordial Magnetic Fields in the Post-recombination Era and Early Reionization
(2005-01-01) Sethi, Shiv K.; Subramanian, Kandaswamy
We explore the ways in which primordial magnetic ﬁelds inﬂuence the thermal and ion- ization history of the post-recombination universe. After recombination the universe becomes mostly neutral resulting also in a sharp drop in the radiative viscosity. Pri- mordial magnetic ﬁelds can then dissipate their energy into the intergalactic medium (IGM) via ambipolar diﬀusion and, for small enough scales, by generating decaying MHD turbulence. These processes can signiﬁcantly modify the thermal and ionization history of the post-recombination universe. We show that the dissipation eﬀects of magnetic ﬁelds which redshifts to a present value B0 = 3 × 10−9 Gauss smoothed on the magnetic Jeans scale and below, can give rise to Thomson scattering optical depths τ > ∼ 0.1, although not in the range of redshifts needed to explain the recent WMAP polarization observations.We also study the possibility that primordial ﬁelds could induce the formation of subgalactic structures for z > ∼ 15. We show that early structure formation induced by nano-Gauss magnetic ﬁelds is potentially capable of producing the early re-ionization implied by the WMAP data. Future CMB observations will be very useful to probe the modiﬁed ionization histories produ
Using Gravitational Lensing to study damped Lyman - alpha clouds
(2000-03-29) Saini, Tarun Deep; Bharadwaj, Somnath; Sethi, Shiv K.
We investigate the possibility of detecting HI emission from gravitationally lensed HI clouds (akin to damped Lyman-α clouds) at high redshift by carrying out deep radio observations in the ﬁelds of known cluster lenses. Such observations will be possible with present radio telescopes only if the lens substantially magniﬁes the ﬂux of the HI emission. While at present this holds the only possibility of detecting the HI emission from such clouds, it has the disadvantage of being restricted to clouds that lie very close to the caustics of the lens. We ﬁnd that observations at a detection threshold of 50 Jy at 320 MHz (possible with the GMRT) have a greater than 20% probability of detecting an HI cloud in the ﬁeld of a cluster, provided the clouds have HI masses in the range 5 × 108 M⊙ ≤ MHI ≤ 2.5 × 1010 M⊙. The probability of detecting a cloud in- creases if they have larger HI masses, except in the cases where the number of HI clouds in the cluster ﬁeld becomes very small. The probability of a detection at 610 MHz and 233 MHz is comparable to that at 320 MHz, though a deﬁnitive statement is diﬃcult owing to uncertainties in the HI content at the redshifts corresponding to these fre- quencies. Observations at a detection threshold of 2 Jy (possible in the future with the SKA) are expected to detect a few HI clouds in the ﬁeld of every cluster provided the clouds have HI masses in the range 2 × 107 M⊙ ≤ MHI ≤ 109 M⊙. Even if such observations do not result in the detection of HI clouds, they will be able to put useful constraints on the HI content of the clouds.